The invention relates to a hydrodynamic bearing for use in a spindle motor of a hard disk drive.
A spindle motor for a hard disk drive generally consists of a rotating component, a rotor, having an annular permanent magnet, and a stationary component, a stator, having a stator stack wound with coils, wherein the rotor is equipped with an appropriate rotary bearing to enable rotor's rotation with respect to the stator.
Alongside roller bearings which have been in use for a long period of time, hydrodynamic bearings are now finding increasing application. A hydrodynamic bearing is an improvement on a journal bearing which consists of a bearing sleeve having a cylindrical inner bearing surface and a shaft having a cylindrical outer bearing surface set into the sleeve. The outer diameter of the shaft is only slightly smaller than the inner diameter of the sleeve, thus creating a radial bearing gap between the two bearing surfaces. The bearing gap is filled with a lubricant, preferably oil, forming a continuous capillary film.
To prevent bearing oil from escaping from the hydrodynamic bearing, one end face of the bearing sleeve is sealed with an airtight seal. At the opposite open end, a concentric area, having, for example, a conical contact surface, can be formed between the shaft and the inner surface of the bearing sleeve. The resulting tapered area functions as both a lubricant reservoir and an oil expansion volume. This tapered area also takes on the function of sealing the bearing. Under the influence of capillary forces, the oil in the area between the shaft and the conical surface of the bearing sleeve forms a stable, continuous liquid film. Therefore, such a seal is typically called a capillary seal.
Such a solution is described in U.S. Pat. No. 5,667,309. Here, a bearing sleeve is disclosed featuring a conical area at its top open end, wherein a concentric tapered area with a rotationally symmetric cross-section is created between the shaft and the bearing sleeve. Bearing oil is contained at the lower end of this tapered area, in direct extension of the bearing gap. The quantity of the bearing oil is so calculated that despite the vaporization of the bearing oil, the bearing gap is always sufficiently filled with bearing oil and seizure of the bearing due to dry running is prevented. The “free” volume of the tapered area, i.e. the volume not filled with bearing fluid, functions as an expansion volume which can be at least partially filled with bearing fluid escaping from the bearing gap due to the fluid volume expansion with a rise in temperature. The concentric tapered area can thus be described as a kind of “overflow” volume which at the same time functions as a lubricant reservoir. The disadvantage of this, in itself simple, solution is that the sealing effect of this tapered area diminishes due to the outward extending cross-section and, as a consequence, its retention capability of the bearing oil also diminishes. Thus, the potential risk that oil is splashed out under axial shock again increases.
A further disadvantage of this known solution is that the useful length of the bearing, and consequently the bearing stiffness, is reduced by the overall axial length of the concentric tapered area conceived as the capillary seal. Here, the overall axial height of the seal cone and the related angle of inclination have to be adjusted to the filling volume and the viscosity of the bearing oil. Low viscosity bearing oils need a more acute angle and thus a larger overall length for the same filling volume.
For fluid bearings which have bearing sleeves with two open ends, the length available for the hydrodynamic radial bearing is reduced equally on both ends of the bearing sleeve. In addition the capability of the bearing to retain the bearing oil film deteriorates due to the cross-section widening at the ends of the bearing sleeve. Consequently, hydrodynamic bearings with a bearing sleeve sealed at one end are basically preferred due to their superior retention capability.
Another disadvantage arises from the fact that at a high rotational speed, considerable centrifugal forces are exerted on the bearing oil. These forces may cause the bearing oil in the tapered area to rise as high as the rim of the bearing sleeve thus increasing the risk of this oil being splashed out.
Since one of the most important criteria for the suitability of hydrodynamic bearings in hard disk drives is the lowest possible bearing power loss, particularly for deployment in portable devices, efforts are made to use bearing oils with the lowest possible viscosity. A capillary seal of the art described adapted to lower viscosity would consequently need a longer overall length which accordingly would have a negative impact on the effective bearing length. Dimensioning a hydrodynamic radial bearing with sufficient stiffness is consequently severely limited, and for very small-scale spindle motors no longer possible under certain circumstances.